Improve errors when marshaling and unmarshaling

[mfcochauxlaberge]

Right now, the functions for marshaling and unmarshaling returns a mix of errors defined in the library (see Error struct and its constructors) and simple Go errors (built with the errors and fmt packages).

The functions for marshaling and unmarshaling (or at least the exported ones) should probably only return errors based on the Error struct.

Investigate and refactor if necessary.